Mono or polycrystalline silicon: who wins the duel?

We know that there are several commercially available photovoltaic technologies. Among them, we can mention photovoltaic crystalline silicon technologies, which include monocrystalline silicon and polycrystalline silicon.

Cultural moment: prefixes of a quantitative nature, such as multi and pluri, are of Latin origin and poly of Greek origin. Therefore, in some texts you may come across the term multicrystalline, which is also correct.

There are several commercially available photovoltaic technologies. We have thin film technologies, which include Amorphous Silicon; Microcrystalline Silicon; Copper, indium and gallium selenide (CIGS), and; o Cadmium telluride (CdTe).

We also have dye-sensitized solar cell technologies and other technologies that are commercialized or are under development. If there are several technologies commercially available, why only talk about monocrystalline and polycrystalline photovoltaic modules?

This is because in 2009, the dominant technology in photovoltaic modules and cells based on mono- and polycrystalline silicon already represented approximately 80% of the global market [1]. Given this scenario, it was up to manufacturers and distributors of photovoltaic modules to decide which photovoltaic technology options to offer to the Brazilian market.

Currently, in the Brazilian market, there are several options for photovoltaic modules, with different manufacturers and different technologies, as well as a wide range of nominal powers. What were the selection criteria for bringing these modules? Based on what requirements?

There are several environmental conditions that affect the output of a photovoltaic power system. These environmental factors must be taken into consideration so that the customer has realistic expectations of the system's output [2].

Below we have two data sheets: on the left the data sheet for a set of polycrystalline modules and on the right the data sheet for a set of monocrystalline modules. All from the same manufacturer and number of cells per module.

The modules were classified according to their maximum rated power. Under standard test conditions (STC), which consider an irradiance of 1000 W/m², absolute air mass of 1.5 and cell temperature at 25°C.

Note that the electrical parameters in both modules, under standard test conditions, vary very little from each other. Furthermore, the efficiencies of modules of the same power are the same.

However, it is enough to leave the standard test conditions for these parameters to change significantly. We can confirm this fact by observing the same datasheet, through data obtained under nominal module operating temperature (NMOT) conditions.

Note that the maximum rated power has been reduced. The 290 Wp polycrystalline modules now have a maximum rated power of 214W and the 290Wp monocrystalline modules now have a maximum rated power of 213W.

These parameters were collected under the modules' nominal operating temperature (NMOT) conditions, that is, irradiance of 800 W/m², absolute air mass of 1.5, ambient temperature of 20°C and air speed of 1 m/s.

Also note that both modules were characterized based on 3 temperature coefficients. The first is directly related to the maximum power of the module, the second to the open circuit voltage and the third to the short circuit current. The nominal operating temperature of the modules is 43 ± 2°C.

Therefore, from the modules' data sheet itself, it is clear that temperature is a parameter that has a great influence on the behavior of a photovoltaic system, since it modifies the system's efficiency and output energy.

Furthermore, atmospheric parameters, such as irradiance level, ambient temperature, wind speed, dirt, dust and the particular conditions of the installation also have an influence [2]. Let's check how the efficiency of the module is evaluated depending on temperature?

Equation 1 presents the instantaneous short circuit current as a function of the current module temperature.

I_SC(T) = ( I_SC + The DT) * S/1000          (1)

Where alpha is the temperature coefficient of the short-circuit current, already seen previously; S is the incident radiation in W/m²; hey_SC is the cell short-circuit current under standard test conditions; and, delta T is the module operating temperature.

The analysis of the module's efficiency is carried out using the Form Factor [3]. Where, the form factor is directly proportional to the maximum voltage point (V_MP) and Maximum current point (I_MP) and inversely proportional to the Open Circuit Voltage (V_O.C.) and short-circuit current (I_SC).

FF = V_MP/V_O.C. x I_MP/I_SC          (2)

To calculate efficiency, we used Equation 3.

H = P_max / P_in =I_SC *V_O.C. *FF/P_in          (3)

We already know that, with temperature variation, the short circuit current changes little, however, the operating voltage of the cells undergoes a large variation. This could be seen directly through the values of the temperature coefficients related to these parameters.

When the temperature increases, keeping the irradiation constant, there is a small increase in the I values_SC and from I_MP, however the values of V_MP and V_O.C. decrease significantly, which results in a reduction of around 0.5%/°C in the module's efficiency.

Photovoltaic modules absorb up to 80% of irradiation. However, only 5 to 20 percent is converted into electricity, depending on the photovoltaic cell technology used. The remainder of this energy is converted into heat.

Due to this effect, on sunny days, photovoltaic modules can reach temperatures of up to 35 degrees above ambient temperature [4]. In hot weather conditions, such as Arizona, module temperatures can reach 85 degrees to 95 degrees Celsius depending on mounting and operating conditions.

In worst-case scenarios, some of the components may reach such high temperatures that they may compromise the safety and functionality requirements of the module and its components [5].

Temperature effects are the result of a natural characteristic of modules based on crystalline silicon cells. They tend to produce greater voltage as the temperature drops and tend to decrease voltage at higher temperatures.

Any photovoltaic system or module must include adjustment calculation due to the effect of temperature [6].

Therefore, one of the characteristics that must be taken into consideration when meeting project requirements is that technical information is provided for standard test conditions, which may never occur in practice.

Second, reliable knowledge of the performance of photovoltaic systems under the real conditions in which they will operate is essential for correct product selection [7].

What does physics tell us?

Well, we have already seen what happens in the field and the explanation of these thermal occurrences based on mathematical models. But what was the mathematical model based on?

Let's see what physics tells us about this phenomenon. We've known since high school physics that when the temperature increases, there are more vibrations in the material's internal lattice. In this case, in the silicon crystal lattice.

Some electrons lose their energy through interactions with these vibrations, rather than contributing to the electrical current. This phenomenon increases as the temperature increases, due to increased vibrations. So we quantify this phenomenon using the temperature coefficient, which tells us how much power is lost for every increase of one degree Celsius.

One of the most important parameters of the material, in relation to this phenomenon, is the order of the atoms in the crystal lattice. Monocrystalline materials are thus characterized by having a perfect recurring order and, therefore, have the highest vibrations and greatest sensitivity to temperature. Polycrystalline materials have only short recurring orders and therefore have moderate sensitivity to temperature. Here we see some examples of polycrystalline networks, where these recurring orders are evident:

Note that there are small recurring orders of crystal lattices. Each crystal grain is limited by boundaries. The boundaries between the grains show the misalignment angle of each crystal lattice. It is also possible to observe that each crystal grain has only one recurring order. If it were a single-crystalline material, there would be no misalignments in the crystal lattice and we would only see a single recurring order. Lately, many monocrystalline modules are featuring in their datasheets, low temperature coefficients in the range of -0.4%/ºC. However, we need to be alert, as physics tells us that the more the material's atoms are placed in strict order, the more the temperature affects it. Let's go back to talking about temperature coefficients again. The average temperature coefficients in monocrystalline silicon modules are about -0.446 percent per degree Celsius and the average temperature coefficients in polycrystalline silicon modules are about -0.387 per cent. cent per degree Celsius [8]. The effect of these differences is very important in hot countries, such as Brazil. As we have already said, a photovoltaic cell under the sun will be much hotter than the ambient temperature. And for its temperature to be in the range of 25 degrees Celsius, which is the temperature under standard test conditions, the ambient temperature must be at almost zero degrees Celsius.

How to interpret temperature coefficients?

Given all this explanation, let's interpret what happens to the electrical parameters of the polycrystalline silicon module in the data sheet shown previously. According to Figure 2, under standard test conditions we have a Maximum Power of 290 W, an Open Circuit Voltage of 38.5 V and a Short Circuit Current of 9.72 A. The module temperature coefficients are:

The temperature coefficient related to the maximum power of the module is equal to -0.39 %/ºC. This means that for every degree Celsius that the module temperature is above 25°C, the module's maximum power will drop by 0.39%. The temperature coefficient related to the open circuit voltage is equal to -0.29 %/ºC, that is, for each degree above 25°C, the open circuit voltage will drop by 0.29%. Finally, the temperature coefficient related to the short-circuit current is equal to -0.05 %/ºC percent per degree Celsius, so the short-circuit current will increase by 0.05% for each degree above 25°C .

Shall we exercise the consequences of what we have seen so far a little more?

Now let's assume the purchase of two similar 100Wp modules. One of them will be monocrystalline silicon and the other will be polycrystalline silicon. Let's consider the temperature coefficient of the monocrystalline module equal to -0.5 %/ºC; and the temperature coefficient of the polycrystalline module equal to -0.4 %/ºC. In the laboratory, when the temperature of the cells is 25 degrees, both will produce 100W. However, in the field, in the middle of summer, the cell temperature will reach around 65 degrees. Then the monocrystalline silicon module will produce 80 W:

100 – [ (65 – 25) * 0.5 ] = 80W

And the polycrystalline silicon module will produce 84 W:

100 – [ (65 – 25) * 0.4 ] = 84W

This will give a difference of 4% compared to the power under standard test conditions. At the end of September 2018, a study carried out by the renowned Fraunhofer Center for Silicon Photovoltaics showed that degradation induced by elevated temperatures leads to a power loss of up to 6% for modules PERC of monocrystalline silicon and these losses for polycrystalline silicon PERC modules are lower than 2% [9]. Therefore, based on practical verifications of the phenomena presented here, mathematical model and the physics involved. We can reach the following conclusions regarding the topic discussed:

• Polycrystalline photovoltaic modules are less sensitive to temperature than monocrystalline modules;
• In hot countries, such as Brazil, it is advisable to use photovoltaic modules that are less sensitive to temperature. Therefore, between a mono and a poly module, the most suitable would be a polycrystalline one;
• If you buy a 340W mono module, in the summer it will output almost as much as a 325W poly module.

References

• [1] Intergovernmental Panel on Climate Change, “Renewable Energy Sources and Climate Change Mitigation – Special Report of the Intergovernmental Panel on Climate Change”, Cambridge University Press, 2012
• [2] Meral, M.E., Dinçer, F., “A Review of the Factors Affecting Operation and Efficiency of Photovoltaic based Electricity Generation Systems”. Yuzuncu Yil University, Turkey, 2011
• [3] Silva, GJ; Silva, WWAG; Reis, GL; Rodrigues, WA, “Evaluation of the Influence of Temperature on the Efficiency of Photovoltaic Panels”. In: VI Brazilian Solar Energy Congress, 2016
• [4] Van Helden WGJ, van Zolingen R. J. Ch, Zondag H. A., “PV Thermal Systems: PV Panels Supplying Renewable Electricity and Heat”, Progress in Photovoltaics: Research and Applications, 2004
• [5] Oh, J., Samy, G., Mani, T., “Temperature Testing and Analysis of PV Modules Per ANSI/UL 1703 and IEC 61730 Standards”, Conference Record of the IEEE Photovoltaic Specialists Conference, Program – 35th IEEE Photovoltaic Specialists Conference, 2010
• [6] Krauter S., “Increased Electrical Yield via Water Fow over the Front of Photovoltaic Panels”, Solar Energy Materials and Solar Cells, 2004
• [7] Li DHW, Cheung GHW, Lam JC, “Analysis of the operational performance and Efficiency characteristic for photovoltaic system in Hong Kong”. Energy Conversion and Management, Vol. 46, No. 7-8, 05.2005, p. 1107-1118
• [8] Dash PK, Gupta NC, “Effect of Temperature on Power Output from Different Commercially available Photovoltaic Modules”, Journal of Engineering Research and Applications, ISSN: 2248-9622, Vol. 5, Issue 1 (Part 1), January 2015, pp.148-151
• [9] R. Gottschalg, M. Pander, M. Turek, J. Bauer, T. Luka, C. Hagendorf, M. Ebert, “Benchmarking Light and Elevated Temperature Induced Degradation (LETID)”, 35th EU PVSEC 2018, 24 – 28 September 2018, Brussels